Generally, the most outwardly visible structures of the human eye include an optically clear anterior cornea, the iris sphincter sitting behind the cornea, and the iris aperture, which is referred to as the pupil. The pupil appears as a circular opening concentrically inward of the iris. Light passes through the pupil along a path to the retina in the back of the eye. In a healthy human eye, a physiological crystalline lens in a capsular bag is positioned posterior to the iris. The chamber between the posterior cornea and the front surface of the capsular bag is commonly referred to as the anterior chamber. A posterior chamber is the area behind the anterior chamber, and includes the capsular bag and physiological crystalline lens.
Ciliary muscle concentrically surrounds the capsular bag, and is coupled to the physiological crystalline lens by suspensory ligaments, also known as zonules. Vitreous humor is contained in the posterior chamber behind the capsular bag. The vitreous humor is surrounded by the retina, which is surrounded by the sclera. The functions and interrelationship of these structures of the human eye are well known in the art and, for this reason, are not elaborated upon in detail herein, except as needed or useful for facilitating an understanding of this invention.
Light entering the emmetropic human eye is converged towards a point focus on the retina known as the fovea. The cornea and tear film are responsible for the initial convergence of entering light. Subsequent to corneal refraction, the incoming light passes through the physiological crystalline lens, where the light is refracted again, towards a point image on the fovea. The amount of bending to which the light is subjected is termed the refractive power. The refractive power needed to focus on an object depends upon how far away the object is from the principle planes of the eye. More refractive power is required for converging light rays to view close objects with clarity than is required for converging light rays to view distant objects with clarity.
A young and healthy physiological lens of the human eye has sufficient elasticity to provide the eye with natural accommodation ability. A young elastic lens may alter its shape, by a process known as accommodation, to change refractive power. The term accommodation refers to the ability of the eye to adjust focus between the distant point of focus, called the Punctum Remotum or pr (far point beyond 20 feet or 6 meters away), and the near point of focus called the Punctum Proximum or pp (near point within 20 feet or 6 meters away from the eye). Focus adjustment is performed in a young elastic lens using the accommodative-convergence mechanism. The ciliary muscle functions to shape the curvature of the physiological crystalline lens to an appropriate optical configuration for focusing and converging light rays entering the eye on the fovea of the retina. It is widely believed that this accommodation is accomplished via contracting and relaxing the ciliary muscles, which accommodate the lens of the eye for near and distant vision, respectively.
More specifically, the eye is “unaccommodated” for far vision by the ciliary muscle relaxing to decrease the convexity of the lens, according to accepted theoretical models of the function of the accommodative mechanism. In this unaccommodated state, the ciliary muscle relaxes. The suspensory zonules holding the lens in place and anchoring it to the ciliary muscle are at their greatest tension. The tension of the zonules causes the lens surfaces to take their flattest curves, making the retina coincident with the far point pr. On the other hand, the ciliary muscle actively accommodates the eye for near vision by increasing the convexity of the lens within the eye via contraction of the muscle. In the accommodated state, the ciliary muscle is constricted in a sphincter-like mode, relaxing the zonules and allowing the lens to take a more convex form. In the fully accommodated state, the retina is coincident with the near point of accommodation pp.
The term emmetropia is understood in the art to mean that natural focus of the optics of the eye when viewing a distant object (greater than 6 meters) is coincident with the retina. The term ammetropia means that the distance focus is displaced from the retina, such as in the case of hypermetropia, astigmatism, and myopia. Hypermetropia denotes an error of refraction caused when the retina intercepts the rays (or pencils) received by the eye before the rays reach their focus. Myopia denotes an error of refraction caused when the pencils within the eye focus to a real point before the pencils reach the retina.
With presbyopia, incoming light rays from the pp are focused at a virtual point situated behind the retina. The ciliary body-zonules-lens complex becomes less efficient at accommodating the focus of these rays on the retina. According to one theory behind presbyopia, the physiological crystalline lens slowly loses its elasticity as it ages. As the physiological crystalline lens ages, the alteration in curvature becomes less for the same action of the ciliary muscle. According to another theory, the physiological lens enlarges with age causing a decrease in working distance between the ciliary body and the lens, resulting in decreased focus ability for the same muscle action. For most people, generally the decline in focusing ability starts in youth and continues until the age of about 60. Generally, it becomes necessary for most people around the age of 40 to use near addition lenses to artificially regain sufficient amplitude at near to accommodate for the pp when attempting to perform near-point activities such as reading. This condition is known as presbyopia, and eventually afflicts almost every human being.
Convergence of the rays in a healthy, phakic (with lens) eye having presbyopia is most commonly achieved with the assistance of eyeglass lenses, contact lenses, or refractive surgery. Once corrected, distance and near objects can be seen clearly.
Aphakia is the condition in which the crystalline lens is either absent or, in very rare cases, displaced from the pupillary area so that it adversely affects the eye's optical focusing system. The former condition may be congenital, but it is usually the result of cataract-removal surgery. With advancing age, the physiological crystalline lens tends to develop opacities—a condition known as cataractogenesis—which unless treated eventually leads to blindness.
In the absence of other pathology or degenerative changes, removal of the opaque crystalline lens afflicted with cataracts restores the possibility of obtaining good vision with refractive implements such as eyeglasses, contact lenses, or intraocular lenses. Pseudophakia describes when the crystalline lens is replaced with a synthetic intraocular lens.
Removal of the crystalline lens by surgery entails the loss of ability to accommodate, so additional positive power in the form of a near addition is needed for near focus. If the synthetic lens is of proper power and results in the pr focusing on the retina, the refractive error for distance will have been eliminated. However, current synthetic intraocular lenses lack the flexibility of a physiological crystalline lens. As a consequence, it is difficult, if not impossible, for the ciliary muscle to focus current synthetic intraocular lenses in the same way as a physiological lens to adjust for objects near the pp. Thus, conventional monofocal intraocular lenses provide little, if any accommodating ability.
Generally, a plus-powered eyeglass lens or contact lens is used in conjunction with an eye having a synthetic intraocular lens to adjust for objects near the pp. Pseudophakic individuals corrected for distance and emmetropia will usually require a lens in front of their eye the equivalent of approximately +2.50 diopters of power to be able to focus on near-point objects between 12 and 20 inches from the eye (approximate). However, “reading” glasses and contact lenses have the drawbacks of being inconvenient, uncomfortable, susceptible to loss and breakage, and in the case of glasses, aesthetically undesirable to some users.
Another problem that adversely affects an individual's eyesight, both near and far, is retinal degenerative condition (RDC). Generally, an RDC involves damage to the macula. An RDC such as macular degeneration leaves the afflicted individual with a “blind spot” or scotoma usually at or near the center of a person's visual field. The afflicted individual is often only able to see peripheral images around the blind spot. The visual field provided by such peripheral images is often insufficient to allow the individual to perform routine activities such as reading, driving a vehicle, or even daily chores and errands. For example, when an individual having a RDC attempts to recognize another person at a distance, the individual may be able to discern the eccentric body portions of the viewed person peripherally, but the scotoma may “wipe out” the facial details of the viewed person, rendering the person unrecognizable.
A person who suffers from a RDC is typically treated optically by using magnification or prism in lens form. A Galilean telescopic magnifying device may be placed in front of the eye or in the eye and customized to the user's needs. The magnification of the device enlarges the image viewed, expanding the image into more healthy areas of retina peripheral (eccentric) to the scotoma. At near, the person suffering from a RDC usually needs magnification in the form of magnifying plus powered lenses and/or prisms—the former (i.e., the plus lenses and magnifiers) to help enlarge the image outside of the scotoma as in the telescopic example and the latter (e.g., the prisms) to help shift the images to different, more functional areas of the retina.
Devices used to provide magnification at distance and near are prescribed according to the art and science of “low-vision”. An example of a low vision device for distance use is a spectacle-mounted telescopic device. An example of a low vision device for near use is a hand-held magnification device and/or prism to assist the user in accessing retinal area peripheral to the damaged area responsible for producing the scotoma. Devices used to provide magnification at distance and near have several drawbacks. First of all, the devices are heavy and bulky, making them difficult to use from an ergonomic perspective. Second, the devices, such as those mounted on a pair of spectacles, may be considered aesthetically unappealing by some. Third, telescopic devices outside of the eye cause distortion (e.g., create aberrations, astigmatism), glare, or decrease the effectiveness of magnification, for example, in the case of spectacle-mounted telescopic devices in which there exists a vertex distance (the distance from the back of the lens to the front of the cornea). Fourth, many current implantable telescopic lenses are held within bulky housings, which decrease the user's peripheral vision and result in a significant loss in the user's field of vision. Fifth, in the example of near vision magnification, the devices are often housed in a hand held device, which prevents the user from accessing “hands free” use of the device—e.g., the user may have trouble holding a newspaper or book in one hand and a device in the other.